Well...speaking as the Electrical Engineer that I am....I researched plugs and plasma so I could be satisfied about the hype. Here's the truth:
1. Precious metal plug tips (platinum, iridium, unobtanium) last longer against spark erosion. Period. No other things are true unless the electrode design changes. Copper center electrodes control heating better than non-copper ones, and last longer, by spreading the heat more quickly across the plug.
2. Electrode design has FAR more to do with boosting "spark performance" than does materials. This is because the tip size of the plug either concentrates the plasma (smaller tips) or spreads the wear over a larger surface (normal plugs). The smaller the "tip" in use, either end of the plug, the easier the spark can concentrate and jump the gap. This is the way plasma works under pressure. The precious metal plugs often (not always) have smaller tips, which improves performance, while the metals reduce the erosion.
3. Coil current controls spark consistency. When points are new, the maximum current flows through the coil during the "charge up" portion of the cycle (points closed). This makes stronger sparks when accelerating. When points deteriorate, this current drops off, and the spark becomes weaker, most noticeably under acceleration and at high RPM. A plug with smaller tips will fire better under these circumstances, so it appears to "last longer" than standard plugs.
4. Resistor plugs should always be used when the coil and wires feeding it are the type designed for resistor plugs. If non-resistor ones are used in this situation, the coils will get extra hot and the wires will begin to burn themselves out, increasing their resistance. The spark duration will be shorter, too, because the plug's current is too high for the coil. Conversely, resistor plugs should NOT be used if the system was not designed for them, as they will reduce the spark's ability to jump the gap. The difference in these systems is in the "tuning" of the inductance of the coil and condenser in use, and they should not be mixed up, lest performance decrease. Some later-model CB750s used resistor-plug systems, K4 and earlier did not.
5. Finally, if you drive a lot in town, seldom at 60+ MPH, try using plugs with "extended reach" tips. In the CB750, this is the "-L" series, like "D8ES-L", or the "-U" series (if these are still around) in the ND plugs. These tips can be measured, if you want to test your findings: stick a thin wire down into the bottom of the insulator and measure its distance. Compare to another plug of the same heat range, but difference "suffix" numbers. For example, if both plugs are the "8" range, but one is the "-L" series, it will be 0.5mm to 0.85mm longer inside.This is a "hotter" tip.
